1. Field of the Invention
The present invention relates to a pattern measuring method and an electron microscope. In particular, it relates to a pattern measuring method for measuring a pattern quickly with high precision and an electron microscope.
2. Background Art
With the recent miniaturization of semiconductor elements, not only the manufacturing equipment but also the inspection and the evaluation equipment are required to be more precise. Scanning electron microscopes (referred to as length measuring SEM, hereinafter) capable of length measurement are commonly used to determine whether the geometrical dimensions of the pattern formed on the semiconductor wafer are appropriate or not.
The length measuring SEM irradiates a wafer with an electron beam, processes the resulting secondary electron signal to form an image, and discriminates the edge of the pattern based on the variations in contrast of the image to derive the dimensions of the pattern (see JP Patent Publication (Kokai) No. 9-166428A (1997) (paragraphs [0012] to [0016] and FIG. 4), for example). To conform to the 35-nm-node design rule, dimension measurement is preferably carried out with an observation magnification of three hundred thousand times or higher. In that case, the entire screen displaying the sample image is equivalent to a square on the wafer having a size of 500 nm by 500 nm or smaller. In order to position a field of view (FOV) at a measurement position within the display screen, the precision of positioning of the field of view has to be on the order of several tens nm at a minimum. However, even when the resolution of the position transducer (laser length measuring machine, for example) of a sample stage is of the order of sub-nanometer, the measuring pattern cannot be positioned at any position within the display screen if the sample stage is inclined or the wafer is displaced on the stage because of some external disturbance.
According to a conventional technique to avoid such a problem, an image is first acquired with a magnification lower than the dimension measuring magnification (ten to twenty thousand times, for example), a characteristic pattern (referred to as addressing pattern, hereinafter) whose positional relationship with the measuring pattern is known is detected from the data about the acquired image, the sample stage is moved or the scanning position of the electron beam is offset based on the position information about the characteristic pattern, thereby precisely positioning the field of view (FOV) for measurement at the measurement position within the display screen with the dimension measuring magnification (see JP Patent Publication (Kokai) No. 3-291842A (1991), for example). According to this technique, even if there are a large number of other patterns closely resembling the measuring pattern in the vicinity of the measuring pattern, the measuring pattern can be surely extracted, and the dimensions thereof can be measured.
In JP Patent Publication (Kokai) No. 9-245709A (1997), there is described a technique of registering as models a plurality of patterns whose relationships with a target position on a sample is known, detecting a pattern similar to any of the registered models, and detecting the target position on the sample based on the position of the pattern and an previously registered offset.
In JP Patent Publication (Kokai) No. 6-283125A (1994), there is described a technique of moving a sample stage to move the field of view in a spiral pattern centered around the initial field of view until a target object is found, in the case where the target object is not found at the target position.